persons of interest or suspects, call them what you will. >> reporter: but carla told investigators she went to her gym, a 30-minute drive from the rintala home, about 3:00 p.m. the afternoon of the murder. said she went out for a long run. then left the gym at 7:00 p.m. investigators located security images that supported her account. is carla's alibi rock solid? or does it have a window for foul play? >> i would say it's pretty rock solid unless she had access to a helicopter. >> reporter: that left investigators with one remaining suspect, the one who'd topped their list since the night of the murder. the person who knew ann best, who loved her, and who fought with her -- cara. because as sunny as these two seemed in public, sometimes behind closed doors they went after one another. as cara admitted in her police interview. >> we'd argue and it would get physical, absolutely. and i'm no angel. but i can honestly say it was definitely back and forth, you know what i mean? >> mm-hmm. >> reporter: their troubles were well documented.